This invention relates to efficient refrigeration systems for closely controlling the cooling regulation of thermal loads which may be required to be held at different temperatures which may be anywhere within a wide range.
In a number of modem applications requiring refrigeration of thermal loads, there is a need for close control at different command temperatures, which may have to be set at widely different temperatures at different times to accommodate a complex process. Some of these applications also require that the cooling system operate without maintenance over a long period of time, while also being compact and requiring minimal floor space. One example of such a demanding application is the cooling of cluster tools in semiconductor fabricating installations, where different subsequences at different times might require cooling the thermal load to temperatures as low as xe2x88x9240xc2x0 C. Downtime caused by support equipment cannot be tolerated, and the fabricating tools are so costly that space is at a premium.
A system which has been proven to meet the somewhat conflicting requirements in a fully satisfactory manner is disclosed in Kenneth W. Cowans patent No. 6,102,113, issued Aug. 15, 2000 and entitled xe2x80x9cTemperature Control of Individual Tools in a Cluster Tool Systemxe2x80x9d. The Cowans system uses high pressure refrigerant with flow controls and thermal energy transfers that are balanced and regulated by the use of temperature and pressure responsive devices at different parts of the refrigeration loop. Reliability and long life operation are enhanced by the use of pressure and temperature responsive valves, and employment of evaporators and heat exchangers which operate stably without deterioration over long periods of time. Internal features are included within the refrigeration loop to guard against excessive or unbalanced temperature and pressure levels to conserve energy. The pressurized refrigerant which is to be expanded to cool the thermal load is passed before expansion through a subcooler and subcooled in counterflow relation to return flows to the compressor. A shunt path between condenser output and the suction line return to the compressor incorporates a desuperheater expansion valve that operates when the compressor approaches too high a temperature to add pressurized refrigerant to the suction line. A pressure responsive hot gas bypass valve also shunts the compressor output to the suction line input in accordance with compressor pressure operating with maximum flow when little or not cooling is required of the system. When such prior system have been required to maintain thermal loads at higher temperatures they have switched to a controlled heating mode, using resistive heating, for example.
However, even greater demands are placed on these systems because of changes demanded in operating of the thermal load, as in more recently developed cluster tools. Whereas earlier systems required controlled heating in an above ambient range, there is now a demand for cooling high temperature loads, at levels up to 120xc2x0 C. This often places unacceptable conditions on a temperature and pressure balanced refrigeration loop. If the expanded gas refrigerant that is to be returned through a subcooler to the suction input of a compressor is at too high a temperature, then the thermal expansion valve which controls refrigerant flow into the evaporator may be supplied a pressurized refrigerant which is partially vaporized. Because the internal mechanism of the thermal expansion valve regulates liquid flow by orifice size, partial vaporization of the liquid renders the device erratic. Consequently, the pressurized refrigerant flow into the evaporator/heat exchanger system becomes unstable and the thermal load cannot be maintained at the selected temperature. This problem cannot be resolved by removing the subcooler, or by eliminating the desuperheater and/or hot gas bypass valve without materially degrading efficiency or performance, and neither enlargement of the power and size of the compressor nor using a separate chiller for the returned expanded gases from the evaporator/heat exchanger is a practical or economically justifiable answer to the problem.
A refrigeration system in accordance with the invention for cooling a thermal load to a selected temperature over a range of xe2x88x9240xc2x0 to 120xc2x0 C. without destabilization employs a refrigeration loop including a subcooler supplying pressurized refrigerant to a thermal expansion valve that regulates chilling of the thermal load in an evaporator/heat exchanger arrangement. The subcooler is used to improve operation at the lowest temperatures, i.e. about xe2x88x9220xc2x0 C. Gaseous refrigerant returning from the evaporator/heat exchanger is passed through the subcooler in counterflow relation to the liquid refrigerant to extract more thermal energy from the liquefied refrigerant to improve system efficiency. A desuperheater expansion valve which responds to high temperature levels at the input to the compressor is coupled to shunt a portion of the output flow from the condenser into the return path for expanded gas through the subcooler. If the thermal load is being cooled in a high temperature range, this diversion of a part of the condenser output to the suction line before the subcooler assures that the counterflow input to the thermal expansion valve remains liquid, while also decreasing the compressor input temperature and increasing the input pressure. This enables the refrigeration system to operate reliably with the thermal load in a high temperature mode, and at a level which would otherwise destabilize the refrigeration loop.
Further in accordance with the invention, stabilization is also improved by shunting a portion of the compressor output to the suction line input in accordance with operating pressure. This shunt path includes a pressure responsive hot gas bypass valve that has a nominal closing threshold of 0 psi, but through its inherent impedance may not shut off except with a differential of about 10 psi. Injection of pressurized refrigerant in the suction line at the subcooler also increases the compressor input pressure and reduces the temperature of the input to the compressor to acceptable levels. In accordance with features of the invention, the desuperheater shunt loop diverts in the range of 0 to 10% of the condenser output to the subcooler depending on the cooling load required of the system and drops the temperature of the cold side of the subcooler to approximately 20xc2x0 C. The hot gas bypass valve diverts approximately 40 to 60% of the compressor output back to the suction line input when fully open. The valve fully opens when no cooling load is required of the system.
Methods of cooling a thermal load in a compressor/condenser system cool high pressure refrigerant delivered to the thermal load using evaporated refrigerant gases after heat exchange with the thermal load. Also, however, they decrease the temperature of the evaporated refrigerant when the thermal load is being chilled at a high level by shunting a portion of the condenser output into the evaporated gases as they are used in subcooling the high pressure refrigerant, thereby to maintain the high pressure refrigerant in liquid state until expansion. Also, when the compressor input is at too low a pressure, the suction line pressure is increased by shunting a portion of the compressor output back to the suction line input.